1. Field of the Invention
The present invention relates to encoding apparatus for business machines such as typewriters, teletypewriters, calculators, adding machines, cash registers, etc. and periheral equipment such as computer terminals, keyboard modules, and the like. More particularly it relates to encoding apparatus for keyboards having sound generating keys coupled with electronics for determining the key depressed by the keyboard operator.
2. Description of the Prior Art
To satisfy the increasing demands and requirements of modern business machines, the trend of manufacturers has been to reduce or eliminate mechanical arrangements for control and data input by introduction of equivalent electronic components and circuitry. The electronic keyboard, in particular, is an approach which is desirable because of the simplicity and reliability of the few moving parts required and elimination of adjustments due to wear. For these reasons, electronic keyboards are more economical because of the significantly reduced manufacturing cost. Another advantage of electronic controls and keyboards in particular is that the space formerly occupied by bulky mechanisms is no longer needed, the great reduction in space requirements thus allowing for a more compact and lighter machine. Also, electronic keyboards easily increase the versatility of business machines because use of larger scale integrated circuits permits increasing functional capabilities at little added cost.
Many prior art business machines employ mechanical switches for sensing mechanical actions, e.g. key depressions in electrical keyboards. This arrangement is undesirable because the switches are subject to wear during use, to atmospheric attack on the contact material and to dirt build up on the contact faces--all of which affect the electrical resistance. To overcome the problem of contact wear and susceptibility to atmospheric attack, manufacturers have had to employ contacts made of a noble metal such as gold at substantial increase in manufacturing expense. The problem of dirt and dust can only be avoided by enclosing the contacts which again adds to the cost.
Bounce is another major problem encountered with the use of switch contacts, it being difficult to eliminate entirely in conventional switches by mechanical design alone. Antibounce circuitry has been introduced as one way to eliminate bounce, but this adds to the complexity of the design with corresponding increase in cost.
The above problems have inspired designs of more elaborate keyboard circuitry employing arrays of devices such as Hall Effect switches, miniature transformers, piezoelectric elements and variable capacitors--many of which eliminate bounce and have a long life. The disadvantages of such circuitry are substantial. The arrays all require many individual transducers mounted on a printed circuit board, one transducer being needed at each key station. Further, these prior art keyboards require many inter-connections as well as many repetitive sensing circuits. For these reasons such keyboard designs have proved complex and expensive.
Another approach in keyboard design is the photo-electric keyboard. Such keyboards typically comprise a matrix of channels and orthogonal grooves with a light source at one end of each channel and a photocell at the other end. Shutters are provided in the grooves for selectively intercepting light beams in the individual channels. Interception of the light beams de-energizes the photocells and, through appropriate circuitry, conveys an information signal. A disadvantage of this keyboard apparatus is that the matrix of channels and grooves is complex and difficult to manufacture. Another disadvantage is that the light sources and photocells must be accurately aligned, which results in costly assembly time.
Accordingly, there is still a need for a low-cost encoding apparatus, one particularly suitable for use with keyboards.